Ramban (Nachmanides) to Genesis 9:12. Ramban: Commentary on the Torah, translated by Rabbi Dr. Charles B. Chavel, Shilo Publishing House, New York, NY, 1971, p. 136

"…Now commentators have said concerning the meaning of this sign that He has not made the rainbow with its feet bent upward because it might have appeared that arrows were being shot from heaven, as in the verse, 'And He sent out his arrows and scattered them' on the earth (Psalms 18:15). Instead, He made it the opposite of this—[with the feet bent downward]—in order to show that they are not shooting at the earth from the heavens. It is indeed the way of warriors to invert the instruments of war which they hold in their hands when calling for peace from their opponents. Moreover, [with the feet of the bow being turned downward towards the eart, it can be seen] that the bow has no rope upon which to bend the arrows." [Brackets inserted by translator.]

Rabbi Shlomo Riskin, Chief Rabbi of Efrat , Commentary on Parshat Noah, 5769.. Available online at http://www.ohrtorahstone.org.il/parsha/5769/noah69.htm

“The 12th century Biblical commentary Ramban (Nahmanides) has a striking explanation for the symbol of the rainbow: ancient cultures fought their wars with the bow and arrow, and the side which surrendered, pursuing peace instead of war, would express their will to do so by raising an inverted bow that the enemy could see. Similarly, G-d places an inverted bow in the heavens as a sign that He is no longer warring against humanity... Whatever the symbolism, it’s clear that the rainbow is a half-picture, lacking a second half to complete the circle of wholeness. G-d can pledge not to destroy humanity, but since He created humanity with freedom of choice, He cannot guarantee that humanity will not destroy itself...”

Rabbi Jonathan Sacks, Chief Rabbi of the United Hebrew Congregations of the British Commonwealth, makes a similar point in his Address to The Lambeth Conference, July 28th ,2008 : available online at http://www.chiefrabbi.org/UploadedFiles/Articals/lambethconference28july08.pdf:

“The covenants of Abraham and Sinai are covenants of faith. But the covenant of Noah says nothing about faith. The world had been almost destroyed by a flood. All mankind, all life, with the exception of Noah's Ark, had shared the same fate. Humanity after the Flood was like the Jewish people after the Holocaust. The covenant of Noah is not a covenant of faith but a covenant of fate. God says: Never again will I destroy the world. But I cannot promise that you will never destroy the world -- because I have given you free will. All I can do is teach you how not to destroy the world. How? The covenant of Noah has three dimensions. First: 'He who sheds the blood of man, by man shall his blood be shed, for in the image of God, He created man.' The first element is the sanctity of human life. The second: Read Genesis 9 carefully and you will see that five times God insists that the covenant of Noah is not merely with humanity, but with all life on earth. So the second element is the integrity of the created world. The third lies in the symbol of the covenant, the rainbow, in which the white light of God is refracted into all the colours of the spectrum. The rainbow symbolises what I have called the dignity of difference. The miracle at the heart of monotheism is that unity up there creates diversity down here. These three dimensions define the covenant of fate.”